1. Field of the Invention
The invention relates to a safety device for electrical storage cell batteries which is more particularly intended to maintain the reliability of the battery in the event of failure of one of the cells, especially in the case of a high-capacity battery made up of series-connected groups of parallel-connected cells.
2. Description of the Prior Art
The conditions under which electrical storage cell batteries are used often require the current and/or the voltage supplied by a battery to a load to be maintained by the battery even if one of its cells becomes unusable, and without the battery being open-circuit at any time. This is the case in particular if it is not possible to repair the battery. It can be achieved in particular when the battery is designed to that end and consists of series-connected groups of parallel-connected cells, for example, as indicated above.
The failed cell has then to be taken out of circuit and this can be achieved in particular by isolating the failed cell from the series connection without at any time rendering the battery open-circuit. This can be achieved by a total shunt applied by means of a conductive device connected in parallel with the failed cell, or where applicable the failed group of cells, to shunt virtually all of the current flowing through the failed cell or failed group of cells.
Shunt devices constructed to this end from diodes or functionally equivalent electronic devices and which shunt the current of a failed cell or a failed group of cells when it leaves a specified range of operating voltage are known in the art. They have the drawback of dissipating a great deal of heat, in particular if they are associated with lithium-ion cells, for which the operating voltage range is from 2.5 V to 4.2 V, and in the case of high-capacity cells using other technologies, especially nickel-hydrogen and nickel-cadmium cells, if the discharge currents are high, for example greater than 80 A.
xe2x80x9cNon-dissipativexe2x80x9d devices used for nickel-hydrogen batteries in particular are also known in the art. They generally have the drawback of applying an external short circuit to the failed cell when they are activated, which is not acceptable in the case of failure of a charged cell. Such failures can be encountered with lithium-ion cells in particular. This is known in the art. To alleviate this drawback, some batteries are equipped with make-before-break devices which first close a circuit in parallel with a failed cell in order to shunt the current flowing in it before breaking the series-connected circuit. These devices isolate the failed cell from the other cells connected in series with it in the battery in a sufficiently short time, routinely of the order of a few milliseconds, for the external short circuit caused in this way not to cause any damage. These devices are complex, however, as they are similar to pyrotechnic devices, and they have the major drawback of including an additional fuse or circuit-breaker component in the series-connected cells or groups of cells of the battery. The component in question therefore reduces the reliability of the system that the battery constitutes.
There is therefore a requirement for a shunt safety device which does not dissipate heat and is suitable for batteries consisting of high-voltage cells, in particular lithium-ion cells, and/or for batteries with a high output current. A shunt safety device is required that can operate with a charged cell without risk of applying an external short circuit to the cell. Furthermore, the device must not include any additional circuit-breaker component that could reduce the reliability of the series-connected circuit of the battery.
The invention therefore proposes a safety device aimed at remedying the above drawbacks, for different electrical storage cell battery configurations, in particular for batteries including a plurality of individual cells connected in series and for batteries consisting of series-connected groups of parallel-connected cells.
The invention provides a safety device for a battery of electrical storage cells composed of modules connected in series each including a storage cell or a group of storage cells and in particular a group of storage cells connected in parallel, the device including at least one individual circuit for individually short circuiting a module if it should fail and maintaining permanently the electrical continuity between the other modules connected in series with it in the battery, wherein the individual circuit includes a first shunt circuit that is connected to the two end terminals of a module in the battery and includes an electrical energy consuming member in series with a switching member for applying a shunt to the terminals of the module via the consuming member if the voltage at the terminals of the module is greater than a particular upper voltage threshold value, the circuit directly short circuiting the terminals of the module if the voltage at the terminals of the module falls below a particular lower voltage threshold value.
In one embodiment of the device according to the invention, said circuit short circuits internally the module to whose terminals it is connected via a first shunt circuit by extending the discharging of said module via said first shunt circuit so that immediately discharging of the battery recommences the battery is discharged until a reversal of polarity occurs which corresponds to a negative threshold of the voltage at the terminals of the module and causing it to be short circuited.
In another embodiment of the device according to the invention a module circuit includes a second shunt circuit connected to the two end terminals of the module in the battery in parallel with the first shunt circuit associated with the module and includes a switching member for short circuiting the terminals of the module directly if the voltage at the terminals of the module is below a particular lower voltage threshold value.
The invention also provides electrical storage cell batteries made up of series-connected modules each including one cell or a group of cells and in particular a group of cells connected in parallel which use a safety device having the above features.
The invention, its features and its advantages are explained in the following description with reference to the accompanying drawings.